A good concentration is 15%. Take half a volume of the acid and half a volume of water and add zinc.
This also works nice with aluminium. Also use 15% for that. The reaction takes quite some time (several minutes) to get started, but once it is going it is become very violent. If you do this with 29% acid and aluminium, then you MUST do it outside, the reaction is really violent and lots of thick smoke of AlCl3 and HCl is emitted. Don't use Al-foil, but Al, used in construction (strips, U and L-shaped profiles, etc.).